The present invention relates in general to radiation detection techniques, and in particular to a new and useful non-destructive in-situ method and apparatus for determining the depth in a medium such as concrete, of radionuclides, between the radioactive source and a detector.
As a result of the Department of Energy""s (DOE""s) shift away from nuclear weapons production and the closing of nuclear power industry facilities, there has been a rapid increase in the number of sites and facilities which contain radioactive contaminants. A large part DOE and nuclear industry activity has been dedicated to the disposition of these facilities through various environmental restoration, and decontamination and decommissioning (DandD) projects. The resources spent on radiological characterization, monitoring and risk assessment contribute significantly to the anticipated total DandD budget of $265 billion dollars over the next 75 years.
Concrete is probably the most important medium or material for radiation shielding and operating structure in nuclear facilities. It has been estimated that there are approximately 73 km3 of radioactively contaminated concrete material within 25 DOE facilities. Additionally the DOE estimates that over 550,000 metric tons of radiological contaminated metal will require characterization and disposition. In many cases, the contaminants are found not only on the surface, but also at depths below the surface of a medium.
Current practice in characterizing deeply contaminated concrete in DOE and nuclear industry facilities employ destructive approaches. In most cases, a surface survey is performed first, and then bore samples are taken manually. The samples are then sent to an off-site lab to analyze the depth profile. It usually takes days to weeks to obtain a result, whose accuracy is only as good as the bore sampling process. For a large contamination area, the task of making representative measurements can be too time-consuming to be practical. Further, because airborne radioactivity is generated when core bore samples are taken manually, occupational exposure is inevitable. The potential for internal exposure to radioactivity requires the use of protective equipment, which in turn slows down the worker""s progress. The current practice in characterizing concrete contamination is inefficient and costly. The radiation protection philosophy of as-low-as-reasonably-achievable (ALARA) can be better accomplished through non-destructive means.
Gamma spectroscopy has been widely used for isotope identification and quantification by measuring the energies of the gamma rays, which can easily penetrate thick samples. When calibrated against a reference radioactive source, the radioactivity of the sample can be determined remotely. Despite these capabilities, gamma spectroscopy has not been demonstrated to be satisfactory for determining a contamination depth profile in-situ. The major difficulty is that while gamma spectroscopy utilizes the photopeaks from the uncollided gamma rays to identify radioisotopes, there has been no sufficient unfolding algorithm to determine the depth to which the gamma rays have penetrated. There is also a lack of tools that can quickly estimate the contamination activity levels and assess the doses or risk caused by the contamination. This information is crucial in deciding cleanup action and demonstrating compliance with regulations on releasing a facility.
Some of the more common contaminants have been reported recently. Table 1 lists the radiological information for some of these contaminants that emit gamma rays. The gamma energy lines are said to be the xe2x80x9cfinger printsxe2x80x9d of these radiological contaminants. For example, it is possible to measure uranium isotopes by detecting the gamma energies, such as the 143 keV (10% yield) and 185 keV (50% yield) for 235U. Highly efficient detectors are often used for isotopes with lower gamma energies and emission yields. The advantage of gamma ray spectroscopy is that gamma rays are much more penetrating than beta and alpha radiation, thus making non-destructive in-situ measurements possible. Table 1 provides the more important gamma lines and gamma yields for common isotopes that may be found in the DOE and nuclear industry. Additionally, 38Cl is listed in Table 1 because the chlorine in salt, NaCl, may be neutron activated and subsequently emit two prominent gamma rays at 1642 keV (32.8% yield) and 2167 keV (44.0% yield).
Items in parenthesis indicate the nuclear reactions producing the radioisotopes based on the bombarding particle and the target nuclei as follows: NTH by thermal neutron isotopes; NFA by fast neutron; CHA by charged particles (alpha, proton, deuterons, etc.); NAT by natural occurring isotopes; NFI by fission with cumulative fission yield in percent for thermal neutron fission of 235U.
The technique of the present invention, as will be explained later in this disclosure, can then be used to determine the depth of salt embedded in pavement or concrete shielding pads. Salt contamination of rebars leads to cracks, potholes, etc. in concrete and pavement.
Many researchers have developed models that use in-situ gamma spectroscopy to estimate the depth of contamination in media. Most of the research has been in the detection of 137Cs contaminant distributions in soil as a result of post-Chernobyl environmental characterizations.
Russ et al. (1996) developed a method using in-situ gamma spectroscopy on transite panels at the DOE Fernald site that required measurements using an uncollimated high purity germanium (HPGe) detector on both sides of the medium. Although the method showed potential for predicting the contaminant depth distribution throughout the thin transite panel, the method requires access to both sides of a medium. Insufficient information was also provide to determine the technique used for any unfolding algorithm that was used. In conjunction with knowledge of the gamma-ray linear attenuation coefficient for the material, the method used the ratio of the photopeak areas at several energies to infer the xe2x80x9cmost-probablexe2x80x9d contamination distribution. Because the method used uncollimated HPGe detectors the in-situ gamma spectroscopy results were of the entire transite panels and local depth profiles across the surface area could not be obtained.
Korun et al. (1991) developed a method to determine depth distribution of 137Cs concentrations in soils based on the energy dependence of attenuation of gamma rays in soil. The method assumed a decreasing exponential distribution in the radionuclide concentration. The decreasing exponential contains a special parameter referred to as the relaxation length. Laboratory and experimental results must be conducted to determine the parameter value. A limitation of the method is that it cannot be applied independently for radionuclides that emit gamma rays at a single energy without prior knowledge about the relaxation length parameter. The method also requires specific knowledge of the linear attenuation coefficients of the materials and the detector""s absolute efficiency. The knowledge of the absolute efficiency is complicated in that the angular dependence of the detector""s efficiency as a function of energy must be known. Korun et al. (1991) recognized that for inhomogeneously distributed radionuclei, the relaxation lengths overestimate the actual depth distribution due to oversimplification in the model.
Fxc3xclxc3x6p and Ragan (1997) improved on Korun""s method for predicting depth using in-situ gamma spectroscopy specifically for 137Cs concentrations in soil. This method makes use of gamma spectroscopy information from the scattered and unscattered gamma rays between the energy range of 0.620 MeV to 0.655 MeV. A limitation of the method is that it requires multiple measurements with and without collimators and it is designed specifically for 137Cs only.
Rybacek et al. (1992) developed a method for depth determination by in-situ gamma spectroscopy. The method used the ratio of fluence rates of unscattered gamma rays of 137Cs whose decay product of 137mBa emits gamma-rays with energies of 0.662 MeV and 0.032 MeV. A limitation of the method is that it requires multiple prominent gamma peaks with large energy differences and was developed primarily for 137Cs.
The methods described above have demonstrated success for using in-situ gamma spectroscopy to determine depth distributions for the specific purposes as designed. Most of the methods have been restricted to the characterization of 137Cs in soil and consequently have limited applicability to DOE and nuclear industry facilities. An appropriate method for DOE and the nuclear industry using in-situ gamma spectroscopy would be one that improves on the limitations of all the methods described above and can be used for many radioactive isotopes such as those listed in Table 1.
An object of the present invention is to provide a method and an apparatus for the non-destructive, in-situ determination of the depth of a radiological contamination in media using gamma spectroscopy and a gamma penetration depth unfolding algorithm (GPDUA) with point kernel techniques to predict the depth of contamination based on the results of uncollided peak information from the in-situ gamma spectroscopy. The invention provides a better, faster, safer and cheaper technique than the current practice for decontamination and decommissioning of facilities that pose a radiation danger. The invention uses a priori knowledge of the contaminant source distribution. The applicable radiological contaminants of interest are any isotopes that emit two or more gamma rays per disintegration or isotopes that emit a single gamma ray but have gamma-emitting progeny in secular equilibrium with its parent (e.g., 60Co, 235U, and 137Cs to name a few)
The predicted depths from the GPDUA algorithm using Monte Carlo N-Particle Transport Code (MCNP) simulations and laboratory experiments using 60Co have consistently produced predicted depths within 20% of the actual or known depth.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.